It's nice to see you here in the Community, corneliakow.
Let me help you go over to your QuickBooks and get this client inactivated. You can review your client's transactions so you can identify why he/she a non-zero balance.
From there, you have the option to delete your customer’s transactions. This way, your client now has a non-zero balance so you can mark it inactive.
You can go through these articles to help you manage your clients as well as your QuickBooks account:
Make a customer inactive and then active
Help Articles for QuickBooks Online
I'm always here to help you out if you need further assistance. I'll be more than willing to lend you a hand.